Microalgal Cells Research Articles

The Effect of Salinity on the Resistance of Plant Cells to Low and Ultra-Low Temperatures on the Example of Dunaliella salina Microalgae Cells

The Effect of Salinity on the Resistance of Plant Cells to Low and Ultra-Low Temperatures on the Example of Dunaliella salina Microalgae Cells

Effects of environmental microplastic exposure on Chlorella sp. biofilm characteristics and its interaction with nitric oxide signaling

Microalgal biofilm is promising in simultaneous pollutants removal, CO2 fixation, and biomass resource transformation when wastewater is used as culturing medium. Nitric oxide (NO) often accumulates in microalgal cells under wastewater treatment relevant abiotic stresses such as nitrogen deficiency, heavy metals, and antibiotics. However, the influence of emerging contaminants such as microplastics (MPs) on microalgal intracellular NO is still unknown. Moreover, the investigated MPs concentrations among existing studies were mostly several magnitudes higher than in real wastewaters, which could offer limited guidance for the effects of MPs on microalgae at environment-relevant concentrations. Therefore, this study investigated three commonly observed MPs in wastewater at environment-relevant concentrations (10–10,000 μg/L) and explored their impacts on attached Chlorella sp. growth characteristics, nutrients removal, and anti-oxidative responses (including intracellular NO content). The nitrogen source NO3−-N at 49 mg/L being 20 % of the nitrogen strength in classic BG-11 medium was selected for MPs exposure experiments because of least intracellular NO accumulation, so that disturbance of intracellular NO by nitrogen availability could be avoided. Under such condition, 10 μg/L polyethylene (PE) MPs displayed most significant microalgal growth inhibition comparing with polyvinyl chloride (PVC) and polyamide (PA) MPs, showing extraordinarily low chlorophyll a/b ratios, and highest superoxide dismutase (SOD) activity and intracellular NO content after 12 days of MPs exposure. PVC MPs exposed cultures displayed highest malonaldehyde (MDA) content because of the toxic characteristics of organochlorines, and most significant correlations of intracellular NO content with conventional anti-oxidative parameters of SOD, CAT (catalase), and MDA. MPs accelerated phosphorus removal, and the type rather than concentration of MPs displayed higher influences, following the trend of PE > PA > PVC. This study expanded the knowledge of microalgal biofilm under environment-relevant concentrations of MPs, and innovatively discovered the significance of intracellular NO as a more sensitive indicator than conventional anti-oxidative parameters under MPs exposure.

Growth inhibition and toxic effects of microplastics on Chlorella vulgaris

Microplastics (MPs) are new pollutants that severely threaten the water environment. This study investigated the effects of three microplastics, namely polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), and polystyrene (PS), on the growth, photosynthesis, biochemical composition, and antioxidant enzyme activity of Chlorella vulgaris. The results show that the average growth inhibition rate of PVC (1000 mg/l) is the highest (29.34 %), followed by PMMA (17.86 %), and PS (13.79 %). The small particle size and high concentration of microplastics are benefited to the inhibitory effect. The increase in soluble proteins within microalgae cells can enhance their water retention capacity and protect cell membranes from mechanical damage and lipid peroxidation caused by microplastics. The decrease in carbohydrate production can be attributed to a decrease in photosynthetic rate and overall physiological activity. The changes in superoxide dismutase and catalase activity can be seen as a defense mechanism for algae cells to maintain antioxidant system balance while removing excess reactive oxygen species.

Identification of odor-causing compounds in six species of odor-producing microalgae separated from drinking water source with distinct fishy odor: Insight into microalgae growth and odor characteristics

Fishy odor, as an offensive and unpleasant odor, could occur in drinking water source with poor nutrition, it is usually considered to be associated with odor-producing microalgae. However, the specific relations among fishy odor, fishy odorants and responsible microalgae were not elucidated comprehensively. In this paper, the odor-causing compounds generated from six microalgae with fishy odor characteristic, isolated in drinking water source Tongyu River, were identified simultaneously. The sensory evaluation result indicated that Tongyu River was principally related to fishy odor (odor intensity 6.5–7.6). Correspondingly, seven, nine, seven, six, seven and seven olfactory detection peaks (ODP) were screened by combining data of GC/O/MS and GC/GC/TOFMS in Cyclotella, Cryptomonas ovate, Melosira, Dinobryon sp., Synedra and Ochromonas sp., which were isolated in Tongyu River and cultured in laboratory. Totally twenty odor-causing compounds, including hexanal, 2-hexenal, 3-hexen-1-ol, heptanal, 1-octen-3-one, 2,4-heptadienal, 2-tetradecanone, 3,5-octadien-2-one, octanal, 1-octen-3-ol, 2-octenal, nonanal, 2,4-octadienal, 2-nonenal, decanal, 2,6-nonadienal, 2-decenal, undecanal, 2,4-decadienal and dodecanal, were screened and identified in all isolated microalgae. Additionally, fishy odor intensity for all identified microalgae increased obviously as microalgae cell number increased and microalgae cell ruptured in cultivation cycles through pearson and spearman correlation analysis. For the first time, twenty odor-causing compounds associating with fishy odor were recognized from six isolated microalgae, which would provide more scientific basis and theoretical support for preventing and treating fishy odor episode of drinking water source.

Photosynthetic H2 production: Lessons from the regulation of electron transfer in microalgae

AbstractGreen hydrogen, produced during microalgal photosynthesis, is regarded as one of the most promising sustainable energy sources. It utilizes sunlight and water, which are essentially unlimited, and its combustion results in only water as a waste product. In microalgal hydrogen energy production systems, the sensitivity of hydrogenase to O2 poses a significant challenge, limiting sustained photosynthetic H2 production in microalgae. Additionally, efficient photosynthetic H2 production in anaerobic microalgal cells is hindered by impaired electron source (photosystem II) and electron loss through the Calvin‐Benson cycle, cyclic electron transfer around photosystem I, and O2 photoreduction, which are identified as the other key challenges. Over the past eight decades, considerable progress has been made in addressing these challenges and regulating electron transfer to achieve sustainable and efficient photosynthetic H2 production in microalgae. In this review, we discuss a range of regulatory methods for achieving sustainable and efficient photosynthetic H2 production in microalgae. Emphasizing the significant progress made over the past eight decades, we also address current challenges and propose potential future solutions.

Mechanism of deep eutectic solvent-mediated microalgal biomass disintegration for enhanced lutein extraction

Microalgae as a source of natural lutein get increasing attention since they feature a high quantity and quality of this pigment. Recalcitrance of the microalgal cell wall declines the bioaccessibility, bioavailability, and digestibility of the beneficial compounds contained in the microalgae. This study aimed to evaluate the influence of deep eutectic solvent (DES) based approaches for the disintegration of Chlorella pyrenoidosa cells for lutein recovery and explore the underlying mechanism. The results showed that choline chloride-ethylene glycol DES (ChCl:EG 1:2) achieved a better cell disintegration and lutein stability than other DESs. Molecular dynamics simulations revealed that chloride anion (Cl−) dominated the dissolution of cellulose for breaking down cell wall structure of C. pyrenoidosa. Solvent screening and process optimization resulted in 239.92 mg/100 g lutein yield with ethanol solvent under optimized condition. Disintegration significantly increased the release of lutein from microalgae. This study provided the theoretical guidance and experimental prove of improve lutein extraction from microalgal biomass in a green and sustainable extraction procedure.

Using microalgae to reduce the use of conventional fertilizers in hydroponics and soil-based cultivation

The intensive use of agrochemicals has led to nutrient loss, greenhouse gas emissions, and resource depletion, thus the development of sustainable agricultural solutions is required. Microalgal biomass has the potential to provide nutrients such as nitrogen, phosphorus, and potassium, along with various plant growth promoters, to enhance crop productivity and impart disease resistance. This study provides a comprehensive assessment of the potential applications of microalgal extracts and biomass in the contexts of seed germination, hydroponic systems, and soil-based crop cultivation. The results revealed that the extracts from Chlorella sp. and Anabaena sp. have no significant impact on the germination of wheat seeds. High concentrations of Chlorella sp. and Anabaena sp. cell extracts in hydroponics enhanced the length of cucumber seedling stems by 81.7 % and 58.3 %, respectively. Additionally, the use of microalgal cell extracts hindered root elongation while stimulating the growth of lateral and fibrous roots. Furthermore, the study compared the performance of 5 different fertilizers: 1) inorganic fertilizer (IF), 2) organic fertilizer (OF), 3) microalgae-based biofertilizer (MF), 4) inorganic fertilizer + microalgae-based biofertilizer (IM), 5) organic fertilizer + microalgae-based biofertilizer (OM). The findings indicate that the plant growth and soil physicochemical properties in the groups supplied with different fertilizers are comparable and significantly higher than those in the control group. The levels of protein, chlorophyll A, and chlorophyll B in the MF group increased significantly by 40 %, 29.2 %, and 33.5 %, respectively, compared to the control group. However, it remained notably lower compared to groups supplied with inorganic and organic fertilizers (p < 0.05). Combining microalgae with organic fertilizer can simultaneously enhance the yield and quality of Chinese cabbage, representing a promising source of crop nutrition. In conclusion, this study suggests that it is promising to use microalgae to reduce the use of conventional fertilizers in hydroponics and soil-based cultivation.

Новый гидрогелевый комплекс с иммобилизованными клетками микроводорослей для удаления аммоний и фосфат-ионов из сточных вод

The aim of the work was to create a stable hydrogel complex containing microalgae cells to remove ammonium and phosphate ions from an experimental solution simulating domestic wastewater. A new polymer matrix based on sodium alginate and carboxymethyl cellulose, polyacrylic acid, retained the integrity of the structure in a solution with a high content of ammonium and phosphate ions, and also proved to be resistant to alkali metals. To increase the stability of hydrogel complexes, we proposed an increase in the number of chemical bonds in the polymer environment due to the crosslinking of alginate and carboxymethyl cellulose with Ca2+ and Fe3+ ions, as wellпа as the introduction of an additional external stabilizer – cationic polyacrylamide. Due to the combination of several polymers, only slight swelling was observed and, as a result, the preservation of the hydrogel matrix in integrity for a long time. The two most commonly used algae in biotesting were selected for the experiment: Tetradesmus obliquus and Chlorella vulgaris from the live microalgae strains collection of the Institute of Biology (Syktyvkar, Russia). The formation conditions of the hydrogel complex made it possible to preserve the viability, reproduction and development of the microalgae cells in the polymer matrix. The cell viability of both cultures was over 95%. It was found that living cells of microalgae are mainly involved in the process of nitrogen uptake. Microalgae immobilized cells, in the presence of carbonate ions, provided the best degree of removal of ammonium nitrogen, which reached 90%. The greatest contribution to the phosphorus removal (&gt; 95%) was made by the polymer component of the hydrogel complex.

Neochloris oleoabundans cell wall rupture through melittin peptide: a new approach to increase lipid recovery.

Microalgae cell wall affects the recovery of lipids, representing one of the main difficulties in the development of biofuel production. This work aimed to test a new method based on melittin peptide to induce a cellular disruption in N. oleoabundans. Neochloris oleoabundans cells were grown at 32°C in the presence of a high concentration of nitrate-phosphate, causing a cell disruption extent of 83.6%. Further, a two-fold increase in lipid recovery following melittin treatment and solvent extraction was observed. Additionally, it was possible to verify the effects of melittin, both before and after treatment on the morphology of the cells. Scanning electron microscopy (SEM) and confocal images of the melittin-treated microalgae revealed extensive cell damage with degradation of the cell wall and release of intracellular material. Melittin produced a selective cell wall rupture effect in N. oleoabundans under some culture conditions. These results represent the first report on the effect of melittin on lipid recovery from microalgae.

Development of a novel Fe@urea nanocomposite for effective harvesting of high lutein-producing microalgae biomass

An Innovative microalgae biomass harvesting is addressed through a newly developed Fe@urea nanocomposite (NC) with high lutein and lipid content. The persistent challenge of scaling up microalgae harvesting for large-scale commercial production is undertaken by coating Fe3O4 nanoparticles (NPs) with urea, significantly enhancing biomass harvesting efficiency. This improvement aligns with increased NH2+ groups on the NC's surface, demonstrating superior performance compared to conventional Fe3O4 NPs, especially at pH 4. Characterization using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), and zeta potential analysis validate the efficacy of the Fe@urea NC. The elemental composition was analyzed through energy-dispersive X-ray spectroscopy (EDX) to confirm effective urea coating on the NC surface. Remarkably, enhanced Fe@urea-mediated harvesting within one minute is achieved through NH2+-assisted interactions. Optimization experiments varying NC concentrations and culture pH, guided by the Langmuir model, showcase the NC's multifaceted solution, excelling under a magnetic field and also via self-flocculation. Cell viability tests confirm lutein-enriched microalgae cell integrity during harvesting at pH 4, preventing product leaching. This synthesized Fe@urea NC presents an efficient and viable method for microalgae harvesting, holding promising implications for large-scale microalgae biorefinery applications.

Development and Application of an Automated Raman Sensor for Bioprocess Monitoring: From the Laboratory to an Algae Production Platform.

Microalgae provide valuable bio-components with economic and environmental benefits. The monitoring of microalgal production is mostly performed using different sensors and analytical methods that, although very powerful, are limited to qualified users. This study proposes an automated Raman spectroscopy-based sensor for the online monitoring of microalgal production. For this purpose, an in situ system with a sampling station was made of a light-tight optical chamber connected to a Raman probe. Microalgal cultures were routed to this chamber by pipes connected to pumps and valves controlled and programmed by a computer. The developed approach was evaluated on Parachlorella kessleri under different culture conditions at a laboratory and an industrial algal platform. As a result, more than 4000 Raman spectra were generated and analysed by statistical methods. These spectra reflected the physiological state of the cells and demonstrate the ability of the developed sensor to monitor the physiology of microalgal cells and their intracellular molecules of interest in a complex production environment.

Insight into the mechanism of nitrogen sufficiency conversion strategy for microalgae-based ammonium-rich wastewater treatment

The strategy of nitrogen sufficiency conversion can improve ammonium nitrogen (NH4+-N) removal with microalgal cells from ammonium-rich wastewater. We selected and identified one promising isolated algal strain, NCU-7, Chlorella sorokiniana, which showed a high algal yield and tolerance to ammonium in wastewater, as well as strong adaptability to N deprivation. The transition from N deprivation through mixotrophy (DN, M) to N sufficiency through autotrophy (SN, P) achieved the highest algal yields (optical density = 1.18 and 1.59) and NH4+-N removal rates (2.5 and 4.2 mg L−1 d−1) from synthetic wastewaters at two NH4+-N concentrations (160 and 320 mg L−1, respectively). Algal cells in DN, M culture obtained the lowest protein content (20.6%) but the highest lipid content (34.0%) among all cultures at the end of the stage 2. After transferring to stage 3, the lowest protein content gradually recovered to almost the same level as SN, P culture on the final day. Transmission electron microscopy and proteomics analysis demonstrated that algal cells had reduced intracellular protein content but accumulated lipids under N deprivation by regulating the reduction in synthesis of protein, carbohydrate, and chloroplast, while enhancing lipid synthesis. After transferring to N sufficiency, algal cells accelerated their growth by recovering protein synthesis, leading to excessive uptake of NH4+-N from wastewater. This study provides specific insights into a nitrogen sufficiency conversion strategy to enhance algal growth and NH4+-N removal/uptake during microalgae-based ammonium-rich wastewater treatment.

Effect of hydrodynamic aeration on bioelectricity generation from photoautotrophic Spirulina platensis in biophotovoltaic devices

A biophotovoltaic (BPV) device is a type of bioelectrochemical cell that shows potential for renewable energy production using photosynthetic exoelectrogenic microorganisms. However, the technology currently has limitations due to its relatively low power output and high dependence on biofilm attachment. Thus, this paper examines the effect of hydrodynamic aeration on the bioelectricity generation of BPV devices. Aeration is widely applied in microalgae cultivation to improve the mixing effect between microalgae cells, culture medium, and air bubbles. The methodology involves the cultivation of Spirulina platensis, device fabrication, voltage measurement in both non-aeration and aeration modes, and the use of polarization curves to determine the device performance under a gradually rising external load. The BPV devices are affected by abnormal voltage drop under high external loads, which is called power overshoot. The use of aeration successfully solved the power overshoot issues over time, but it also resulted in a gradual reduction in the voltage output and power density of the BPV devices. During aeration, the BPV devices produced a peak power density of 0.121 ± 0.056 mWm−2 on Day 1, which declined to 0.048 ± 0.025 mWm−2 on Day 13. The blowing of air bubbles had some side effects on BPV devices, including causing the attachment of Spirulina cells to the container walls and accelerating the evaporation of the culture medium. This study emphasizes the role of hydrodynamic aeration in the bioelectricity generation process and opens up possibilities for wider application of BPV devices in renewable energy production.

Simultaneous Separating, Splitting, Collecting, and Dispensing by Droplet Pinch-Off for Droplet Cell Culture.

Simultaneous separating, splitting, collecting, and dispensing a cell suspension droplet has been demonstrated by aspiration and subsequent droplet pinch-off for use in microfluidic droplet cell culture systems. This method is applied to cell manipulations including aliquots and concentrations of microalgal and mammalian cell suspensions. Especially, medium exchange of spheroid droplets is successfully demonstrated by collecting more than 99% of all culture medium without damaging the spheroids, demonstrating its potential for a 3D cell culture system. Through dimensional analysis and systematic parametric studies, it is found that initial mother droplet size together with aspiration flow rate determines three droplet pinch-off regimes. By observing contact angle changes during aspiration, the difference in the large and the small droplet pinch-off can be quantitatively explained using force balance. It is found that the capillary number plays a significant role in droplet pinch-off, but the Bond number and the Ohnesorge number have minor effects. Since the dispensed droplet size is mainly determined by the capillary number, the dispensed droplet size can be controlled simply by adjusting the aspiration flow rate. It is hoped that this method can contribute to various fields using droplets, such as droplet cell culture and digital microfluidics, beyond the generation of small droplets.

Computational fluid dynamics coupled to biokinetic models: Numerical methodology for microalgae cultivation optimization

Mixing of the algal culture medium in a photobioreactor (PBR) is a crucial factor to enhance microalgae cultivation and production, since it ensures microalgae cells can homogeneously access nutrients and light radiation. Hydrodynamic conditions in an industrially applicable PBR cannot be tested in laboratory cultivation systems since the scale-up methodology is not specified; hence requiring complex tests at a demonstrative scale or the application of mathematical models. In this study, a three-dimensional numerical model of the hydrodynamic conditions in a hybrid horizontal tubular PBR was developed to investigate the mixing of the culture medium under various operating conditions, in order to optimize biomass production in this type of system. The PBR consists of two open tanks connected through sixteen polyethylene transparent tubes, at a demonstrative scale (total volume 11.7 m3). Microalgae cells movement inside the tubes was simulated. Cell trajectory prediction allows to simulate the intensity of mixing of the culture medium. Subsequent coupling with the biokinetic BIO_ALGAE model allows to monitor the influence of hydrodynamic conditions on the distribution of light in the culture medium and the yield of microalgae. The simulations were experimentally validated. The more intense mixing of the culture medium caused by increasing flow rate (Re = 23,700–46,200) allows more frequent exposure of the microalgae cells to the light source, which according to the results obtained can increase the microalgal yield by 14% in the summer cultivation period and by 151% in the winter period.

One-step pyrolysis-catalytic hydrogen production by the oleaginous microalga Chromochloris zofingiensis

In this study, one-step pyrolysis-catalytic hydrogen production was developed with the substance of a typical oleaginous microalga, Chromochloris zofingiensis, with Ni/Zr0.2Ce0.8O2 as the catalyst. Our results demonstrated that the formation of a ZrO2–CeO2 solid provided thermal stability to the catalyst and showed abundant active sites for catalysis. Moreover, 450 °C was the optimum temperature for hydrogen production by microalgae. At this temperature, hydrogen production reached 25.47 mmol g−1, with hydrogen accounting for 43.5 % of the total gas, comparable to results previously reported for “two-step” hydrogen production methos. Further studies showed that lipids within microalgal cells were the major substance for hydrogen production, whereas starch and protein had limited hydrogen-producing capabilities. Therefore, by increasing the lipid content to 33.1 % of cell dry weight through abiotic stress, the hydrogen yields significantly improved, reaching 47.71 mmol g−1, with hydrogen constituting 69.4 % of the total gas. In summary, our study proved that oleaginous microalgae were feasible feedstock for one-step pyrolysis-catalytic hydrogen production.

Effects of Light Intensity on the Growth and Biochemical Composition in Various Microalgae Grown at High CO2 Concentrations.

In modern energy, various technologies for absorbing carbon dioxide from the atmosphere are being considered, including photosynthetic microalgae. An important task is to obtain maximum productivity at high concentrations of CO2 in gas-air mixtures. In this regard, the aim of the investigation is to study the effect of light intensity on the biomass growth and biochemical composition of five different microalgae strains: Arthrospira platensis, Chlorella ellipsoidea, Chlorella vulgaris, Gloeotila pulchra, and Elliptochloris subsphaerica. To assess the viability of microalgae cells, the method of cytochemical staining with methylene blue, which enables identifying dead cells during microscopy, was used. The microalgae were cultivated at 6% CO2 and five different intensities: 80, 120, 160, 200, and 245 μmol quanta·m-2·s-1. The maximum growth rate among all strains was obtained for C. vulgaris (0.78 g·L-1·d-1) at an illumination intensity of 245 µmol quanta·m-2·s-1. For E. subsphaerica and A. platensis, similar results (approximately 0.59 and 0.25 g·L-1·d-1 for each strain) were obtained at an illumination intensity of 160 and 245 µmol quanta·m-2·s-1. A decrease in protein content with an increase in illumination was noted for C. vulgaris (from 61.0 to 46.6%) and A. platensis (from 43.8 to 33.6%), and a slight increase in lipid content was shown by A. platensis (from 17.8 to 21.4%). The possibility of increasing microalgae biomass productivity by increasing illumination has been demonstrated. This result can also be considered as showing potential for enhanced lipid microalgae production for biodiesel applications.

Inhibition mechanism and biological recycling utilization of microalgae culture aging solution

During the process of recovering microalgae culture media, the culture media will age and inhibit the growth of microalgae. When the culture media was reused for the third time, the maximum biomass of Spirulina platensis FACHB-439 was only 56.13% of the fresh culture media. But the inhibitors in the aging solution are not known. In this study, the growth inhibition mechanism of Spirulina platensis aging solution was clarified through the identification of target organic matter by cyclic culture experiment and ion chromatography. The results showed that the growth inhibitors in the aging solution were polysaccharides, mainly including fucose and rhamnose. Further reverse addition experiments confirmed that polysaccharide inhibitors would increase with the increase of repeated use of the culture media. By inhibiting the synthesis of photosynthetic pigments in microalgae, they could cause oxidative damage and membrane peroxidation of microalgae cells, resulting in stress on the growth of microalgae. At the same time, this study established a purification method for reusing aging solution of heterologous microalgae (Chlorella sp. L166) to obtain high lipid and high carbohydrate biomass energy. The results showed that the biomass of Chlorella sp. L166 in the first circulating aging solution of S.platensis FACHB-439 increased by 25%, the carbon utilization rate reached 21.36%, the lipid content per cell increased by 1.61 times, and the carbohydrate content increased by 86.71%. This work is helpful to solve the problem that aging of culture media inhibits carbon sequestration efficiency and biomass yield of microalgae in large-scale production of microalgae. And the results of this study have achieved high utilization rate of water resource for large-scale cultivation of microalgae, which is conducive to realize the scale development of microalgae biotechnology.

Regulation effects of indoleacetic acid on lipid production and nutrient removal of Chlorella pyrenoidosa in seawater-containing wastewater

The utilization of seawater supplemented with wastewater nutrients for microalgae cultivation represents a promising and cost-effective approach that combines the benefits of wastewater treatment and microalgal resource recovery. However, the high salt content in seawater poses a significant challenge, hindering microalgal growth and reducing the removal of nitrogen and phosphorus on a large scale. The phytohormone indoleacetic acid (IAA) was used in this study to enhance stress resistance and lipid production of Chlorella pyrenoidosa grown in seawater-wastewater medium. Compared to the control groups involving regular wastewater and seawater-containing wastewater without IAA, Chlorella pyrenoidosa cultivated in the seawater-containing wastewater supplemented with IAA exhibited remarkable outcomes. Specifically, microalgae in IAA-enhanced seawater-containing wastewater achieved the highest lipid productivity (22.67 mg L−1 d−1) along with impressive nitrogen (99.3 %) and phosphorus (97.3 %) removal rates. Moreover, their cell sedimentation ratio reached 76.6 %, indicating enhanced settling properties. Additionally, the physiological mechanism changes after exposure to seawater stress and IAA were revealed based on the changes in antioxidant enzymes, endogenous hormones, and fatty acid saturation. Furthermore, the transcriptomic analysis elucidated the molecular mechanisms underlying microalgal lipid synthesis and their response to antioxidant stress when exposed to seawater. The supplementation of IAA under seawater stress stimulated energy metabolism and the antioxidant response in microalgal cells, effectively mitigating the adverse effects of seawater stress and promoting overall algal lipid productivity. Overall, this study unveiled the potential of exogenous plant hormones, particularly IAA, in enhancing stress resistance and lipid productivity of microalgae grown in seawater-wastewater medium, which significantly contributed towards the efficient use of seawater resources for microalgae cultivation and biofuel production.

Behavior and surface properties of microalgae indicate environmental changes

We conducted a microcosm experiment under near-natural conditions to investigate the effects of environmental stressors temperature and salinity on the response of three microalgal species. The response of microalgae to increased temperature and altered salinity is species-specific, with cell barrier complexity playing an important protective role. At decreasing salinity, the green flagellate cells of Dunaliella tertiolecta vibrate around the point and become stiff while their physiological activity is at its highest, which may indicate that organic matter is being released into the medium. At the highest salinity, Tetraselmis suecica showed the highest growth rate along with the hydrophobic behavior of the cells, which is a favorable condition for the formation of aggregates. Under these conditions, the cells could enter the cyst stage, while the content of polar lipids could mediate cell adhesion to the natural interfaces. Cylindrotheca closterium, a diatom with an organosilicate cell wall, also showed the highest physiological activity at decreasing salinity. However, the cells were softest and exhibited hydrophilic behavior, implying that released organic material could alter the surface properties of the diatom. This study provides insight into the behavioral, mechanical, and molecular changes of microalgae under near-natural conditions, which may contribute to a fundamental understanding of how biochemistry and mechanics interact to control the functional behavior of microalgal cells in climatically altered aquatic systems.